如何将MKOverlay对象存储到文件以供稍后检索

Question

我正在编写一个iOS应用程序来记录您的旅行路径，然后将其存储起来以便稍后可以检索它。绘制路径的大部分代码都基于示例代码Breadcrumb。

现在我正在添加功能以保存绘制的叠加层。什么是最好的方式来做到这一点？我可以使用CoreData，但我不希望对绘制的叠加层做很多工作，而不是稍后再检索它。我目前已经尝试了一个简单的NSArray。我将CrumbPath对象转换为NSData并将其存储在数组中。稍后我会检索它并将其转换回CrumbPath。但是，我似乎做错了什么。

@interface CrumbPath : NSObject <MKOverlay> 
{ 
    MKMapPoint *points; 
    NSUInteger pointCount; 
    NSUInteger pointSpace; 

    MKMapRect boundingMapRect; 

    pthread_rwlock_t rwLock; 
} 

- (id)initWithCenterCoordinate:(CLLocationCoordinate2D)coord; 
- (MKMapRect)addCoordinate:(CLLocationCoordinate2D)coord; 

@property (readonly) MKMapPoint *points; 
@property (readonly) NSUInteger pointCount; 

@end 

我节省CrumbPath对象 “面包屑” 是这样的：

NSData *pointData = [NSData dataWithBytes:crumbs.points length:crumbs.pointCount * sizeof(MKMapPoint)]; 
[patternArray addObject:pointData]; 
[timeArray addObject:date]; 

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES); 
NSString *documentsDirectory = [paths objectAtIndex:0]; 
// Create the full file path by appending the desired file name 
NSString *patternFile = [documentsDirectory stringByAppendingPathComponent:@"patterns.dat"]; 
NSString *timeFile = [documentsDirectory stringByAppendingPathComponent:@"times.dat"]; 

//Save the array 
[patternArray writeToFile:patternFile atomically:YES]; 
[timeArray writeToFile:timeFile atomically:YES]; 

后来这样获取在表中显示：

NSData *pointData = [appDelegate.patternArray objectAtIndex:indexPath.row]; 
MKMapPoint *points = malloc(pointData.length); 
(void)memcpy([pointData bytes], points, sizeof(pointData)); 

并再次构建路径：

crumbs = [[CrumbPath alloc] initWithCenterCoordinate:MKCoordinateForMapPoint(points[0])]; 
for (int i = 1; i <= pointCount; i++) { 
    [crumbs addCoordinate:MKCoordinateForMapPoint(points[i])]; 
}  
[map addOverlay:crumbs]; 

但是，我得到一个错误：'NSInvalidArgumentException', reason: '-[NSConcreteData isEqualToString:]: unrecognized selector sent to instance

Answer 1

就个人而言，我倾向于做如下：

1. 首先，我会倾向于节省CLLocationCoordinate2D C数组坐标，我传递给MKPolygon实例方法polygonWithCoordinates（而不是一个MKMapPoint C数组）。如果您愿意使用MKMapPoints，则可以修改此代码，但我更喜欢一种格式，我可以从外部检查并理解（即CLLocationCoordinate2D的经度和纬度值）。所以，让我们假设你让你MKPolygon，象这样一行代码：

   MKPolygon* poly = [MKPolygon polygonWithCoordinates:coordinates count:count]; 
   

   你可以因此节省坐标，像这样：

   [self writeCoordinates:coordinates count:count file:filename]; 
   

   凡writeCoordinates:count:filename:定义如下：

   0123：

   - (void)writeCoordinates:(CLLocationCoordinate2D *)coordinates count:(NSUInteger)count file:(NSString *)filename 
   { 
       NSData *data = [NSData dataWithBytes:coordinates length:count * sizeof(CLLocationCoordinate2D)]; 
       [data writeToFile:filename atomically:NO]; 
   } 
   

   然后，您可以用使MKPolygon从文件

   其中polygonWithContentsOfFile定义为：

   - (MKPolygon *)polygonWithContentsOfFile:(NSString *)filename 
   { 
       NSData *data = [NSData dataWithContentsOfFile:filename]; 
       NSUInteger count = data.length/sizeof(CLLocationCoordinate2D); 
       CLLocationCoordinate2D *coordinates = (CLLocationCoordinate2D *)data.bytes; 
   
       return [MKPolygon polygonWithCoordinates:coordinates count:count]; 
   } 
   

2. 或者，也可以读出并在一个的plist格式写的CLLocationCoordinate2D阵列（这使其在人类可读的格式），通过替换上述两种方式搭配：

   const NSString *kLatitudeKey = @"latitude"; 
   const NSString *kLongitudeKey = @"longitude"; 
   
   - (void)writeCoordinates:(CLLocationCoordinate2D *)coordinates count:(NSUInteger)count file:(NSString *)filename 
   { 
       NSMutableArray *array = [NSMutableArray arrayWithCapacity:count]; 
   
       for (NSUInteger i = 0; i < count; i++) 
       { 
        CLLocationCoordinate2D coordinate = coordinates[i]; 
        [array addObject:@{kLatitudeKey:@(coordinate.latitude), kLongitudeKey:@(coordinate.longitude)}]; 
       } 
   
       [array writeToFile:filename atomically:NO]; 
   } 
   
   - (MKPolygon *)polygonWithContentsOfFile:(NSString *)filename 
   {   
       NSArray *array = [NSArray arrayWithContentsOfFile:filename]; 
       NSUInteger count = [array count]; 
       CLLocationCoordinate2D coordinates[count]; 
   
       for (NSUInteger i = 0; i < count; i++) 
       { 
        NSDictionary *dictionary = array[i]; 
        coordinates[i].latitude = [dictionary[kLatitudeKey] doubleValue]; 
        coordinates[i].longitude = [dictionary[kLongitudeKey] doubleValue]; 
       } 
   
       return [MKPolygon polygonWithCoordinates:coordinates count:count]; 
   } 
   

显然你需要添加错误校验码，以确保该writeToFile和dataWithContentsOfFile（或arrayWithContentsOfFile）成功，但希望这给你的想法。